The proposed research will provide a 5-year longitudinal study of the development of arithmetic-procedural competencies and number estimation skills in children with a learning disability in mathematics (MD) and normal peers. It is well established that during the solving of simple arithmetic problems (e.g., 6+7) MD children 1) rely on developmentally immature strategies, such as finger counting;2) frequently commit counting errors;3) use immature counting procedures [e.g., they count both addends rather than stating the cardinal value of one addend and counting-on a number of times equal to the value of the other addend];4) and, have difficulties retrieving basic facts from long-term memory. The focus of our currently funded research is to identify the basic cognitive systems, such as working memory, that underlie these arithmetic deficits. We have found, for instance, a strong relation between working memory and MD children's reliance on finger counting and counting errors, and a relation between their use of immature counting procedures and knowledge of counting concepts. In other words, MD children have a lower working memory capacity than their normal peers, and this group difference in working memory explains part of the group differences in use of finger counting and counting errors. Group differences in knowledge of counting principles, in contrast, is unrelated to use of finger counting or counting errors, but explains part of the group difference in the type of procedure used during counting. The proposed research will provide the first longitudinal assessment of these relations between MD children's arithmetic deficits and underlying cognitive systems, and will expand the assessments to include more complex arithmetic problems than have been used in most previous studies. The proposed research will also be the first to assess MD children's ability to use spatial representations (i.e., a mental number line) to make number estimations, and assess how the form of the representation changes with formal schooling. The goals are to document the developmental progression of arithmetic and number estimation competencies in MD children, and to identify - based on the theoretical rationale described in the Background and Significance Section - the cognitive systems (e.g., working memory) that underlie MD children's developmental delays and deficits in these competencies.